False Retrodiction

Some Comments on the Misuse of
Planetarium Software for Analysing Ancient Events

There is a growing trend, probably triggered by the availability of
excellent planetarium software at little or no cost, for this to be misused in
relation to ancient events. This short article examines this misuse in general.

It is usually the case that the
people who are guilty of this are woefully ignorant of the limitations of the
software they use and, consequently their claims are specious at best. They most
often fall into the camps of pseudo-historians or pseudo-archaeologists (they
themselves prefer the term "alternative" to "pseudo"). They
are part of a growing trend to misuse astronomy to justify outrageous claims for
which there is no tangible evidence.

The better amateur planetarium programs
use the full VSOP87 (Variations Seculaires des Orbites Planetaires)[1] theory
to calculate planetary positions over long periods [2]; lesser programs use less
precise methods. The accuracy of VSOP87 probably deteriorates with
increasing time; it is thought to be precise to better than an arcminute
for dates later than ab out 6000 years BP. The point is that no-one actually
knows how accurate it is, even as far back as that. Prior to 6000BP ,
the errors increase markedly — the accuracy of all planetary theories does
this outside their proper time range.

Another factor affecting the observed position of solar system objects in
the skyor on the horizon is the obliquity of the ecliptic. Bill Gray, the author
of Guide, warns against using his
program outside the range of 10,000 years either side of the present because the
rapidly increasing errors outside this range.

The retrodiction [3] of cometary positions into the past is fraught with
difficulties. Comets are relatively low-mass objects whose orbits have a high
eccentricity. If a comet passes close to a planet, its trajectory will be
changed. (In the case of Comet Shoemaker-Levy 9, it was changed so that
it impacted the planet!) Hence, the retrodiction of cometary positions for
periods in excess of a couple of millennia is generally foolhardy at best.

[1] Full precision VSOP87 (and PS-1996 [2]) is definitely used by
Guide and
Skymap; most other planetarium programs
use a lower-precision model from Jean Meeus's
Astronomical
Algorithms. [2] PS-1996 theory is used over shorter periods —
up to about 100 years for the naked-eye planets.[3] Backward prediction.